1. Field of the Invention
This invention relates generally to web processing machines, e.g., paper machines, coaters and the like, and is concerned in particular with an improved apparatus for doctoring the rolls of such machines.
2. Description of the Prior Art
The three main components of a doctor system include a rigid doctor structure or beam, a doctor blade holder, and a doctor blade. The doctor system is adapted to be mounted in a paper machine, coater or other like web processing machine and is used to scrape the rotating rolls for cleaning or dewatering purposes. At some critical roll positions in the paper machine, the doctor system is also used during the threading process to remove or "shed" either a full width sheet of paper or a portion of the sheet.
In conventional doctoring, the working edge of a doctor blade is applied to the rotating roll surface from which the material, water or other solutions are to be removed. The critical factors contributing to an effective doctoring operation include a uniform load between the working edge of the blade and the rotating roll surface for the full length of the blade, and maintenance of an optimum blade angle with respect to the roll surface.
FIGS. 1-3 illustrate examples of prior art doctoring systems. In FIGS. 1 and 2, a doctor system includes a doctor back 10 mounted in bearings 12 for rotation about an axis A.sub.1. An actuator 14 operates to rotate the doctor back about axis A.sub.1 in order to load the working edge of a doctor blade 16 against the surface 18 of a rotating roll 20. A blade holder 22 is mounted to the front of the doctor back 10.
In the following description, it is to be understood that the doctor blade and the principal associated components of the doctoring system, e.g., the doctor back and blade holder, each extend in the "cross machine direction", i.e., parallel to the rotational axis A.sub.1 of roll 20, for at least the full width of the web being processed. Moreover, as herein employed, the term "machine direction" is to be understood as referring to the direction of travel of the web through the machine.
As can best be seen in FIG. 2, the blade holder includes a relatively rigid top plate 24 and an underlying jaw 26. The doctor blade 16 is removably received in a slot 28 where it is supported between the top plate 24 and the jaw 26.
The top plate is fastened to the doctor back by threaded screws 30 or the like. Adjustable set screws 32 are provided to adjust the blade holder and doctor blade along the length of the roll 20 in order to accommodate localized roll surface variations. Even though the set screws 32 provide some measure of adjustability, achieving a uniform load between the blade working edge and the varying roll surfaces continues to be problematical. The main drawback of this type of apparatus is that because the doctor blade 16 is held between the top plate 24 and the jaw 26, both being relatively rigid members, there is a lack of flexibility needed to satisfactorily accommodate the often encountered roll surface irregularities.
FIG. 3 illustrates a more advanced prior art doctoring system. Here, a blade holder 34 is again mounted to the doctor back 10. A top plate 36 is pivotally mounted for rotation about an axis A.sub.2 defined by a pivot pin 38 extending for the full length of the holder. The pivot pin is threaded through top brackets 40 depending from the top plate 36, and similar base brackets 42 extending upwardly from a channel-shaped base 43. A jaw 44 is mounted to the underside of the top plate 36 in a forward position. The jaw 44 together with the top plate 36 defines a slot 46 in which the doctor blade 16 is removably received for application to the surface 18 of roll 20.
An air or liquid filled load tube 48 extends for the full length of the holder and lies parallel to the pivot pin 38 in a position furthest from the doctor blade 16. The load tube 48 is either inflated to generate a doctor blade loading force, or it provides a reactionary force with the top plate 36 when the doctor back 10 is rotated towards the roll surface 18.
An additional air filled tube 50 is located parallel to the pivot pin 18 in a forward position. When it is necessary to unload the doctor blade 16, either the air filled tube 50 is inflated while the load tube 48 is deflated, or the doctor back 10 is rotated away from the roll surface 18. The air filled tube 50 also creates a seal to prevent debris from entering the holder cavities.
The main disadvantage of this type of doctoring system is that friction between the pivot pin 38 and the top plate brackets 40 along with a relatively low mechanical advantage provided by the load tube 48 combines to make the contact load between the doctor blade and the roll surface somewhat variable and unpredictable. An additional disadvantage is that the necessary clearances present between the pivot pin 38 and the associated brackets 40, 42 makes the doctoring apparatus susceptible to vibration and chatter problems.
Examples of other prior art doctoring arrangements are provided by U.S. Pat. Nos. 4,789,432; 4,367,120; and 3,748,686.